1. Field of the Invention
The field of the present invention relates to medical diagnostic techniques generally, and more particularly to a method for measuring the dimensions of a vessel. The term “vessel” refers to the vessels and ducts of the human body, namely the blood vessel and the ducts such as the urethra that convey other liquids. An aim of the invention is to provide greater knowledge of the size and geometry of these vessels and ducts in order to facilitate subsequent therapeutic action. One of the most immediate applications of the invention is in the insertion of stents. Naturally, other uses of the dimension-measuring method of the invention can be envisaged.
2. Description of the Prior Art
A stent is a small metal spring that is slid into a natural human cavity in order to keep it open. It is used essentially in arteries in angioplasty. A stent can also be used in the urethra, the bile ducts etc. Angioplasty is a procedure used to dilate a narrowed artery by means of a balloon that is inflated in this artery, thus crushing an atheromatous plaque that has caused a stenosis. The main drawback of this technique is the high rate of restenosis, i.e. the repetition of the narrowing process which occurs in almost half of the cases. This restenosis may be premature (the elastic return of the artery after the balloon has been deflated) or delayed by proliferation of cells in the wall (the endothelium) of the artery.
To be put in place, the stent is positioned on a folded angioplasty balloon. The unit is conveyed into the vessel until the place of intervention. When the balloon is inflated in the artery, the spring expands and prevents the elastic return of the stenosis. The balloon is then withdrawn and the stent remains in place. It may be positioned by direct stenting or after dilation by a first balloon. The positioning of the stent is done under radioscopy and does not appreciably lengthen the angioplasty procedure.
Since the stent is foreign to the human body, it is a natural point of focus for the formation of a clot. A clot-prevention treatment therefore remains indispensable for at least several weeks, until the metal is naturally coated with the cells of the internal walls of the artery, in a process of endothelialization. This treatment is based on aspirin in small doses, historically associated with ticlopidine. At present, the preferred association consists of aspirin and clopidogrel, two platelet antiaggregants. The stent very appreciably diminishes the frequency of restenosis after angioplasty. It is very commonly used during angioplasty. There is no allergy. Although made of metal, it does not hamper an IRM operation.
A stent is characterized by its diameter (once unfolded) and its length. The problems posed by stents are essentially related to the need to know their exact dimensions, especially when the part of the duct in which they have to be implanted is curved. In practice, since the vessel has a tube-like structure, it is necessary to know the longest dimension of the stent and its shortest dimension so as to foresee its average curvature and hence its implantation.
The measurement of the vessels in the body by 3D imaging is now a well-established medical procedure. This procedure is leading to a simplification of surgical operations and radiology operations. There are numerous medical image-processing techniques to help in this task. The applications most commonly used serve to determine the size and geometry of a prosthetic device. In particular, they serve to measure the lengths, diameters and volumes to prepare a positioning of the stent in a vessel, such as the coronary artery, the carotid, the iliac artery, etc. that has undergone stenosis.
The technique currently used for sizing stents is to determine the central line in the vessels, and measure the length of this central line. Such an approach naturally implies that the vessel to be measured is straight, or at least that it will become straight, after the prosthesis has been put in place. However, such an assumption is not valid for certain vessels. For, it is increasingly common to place stents in a portion of the thoracic artery that may have a curvature of more than 90 degrees. When deployed, the shape of the stent adapts to the shape of the aorta in being curved. In this case however, in order to choose the appropriate stent, it is very important to know its exact dimensions.
To resolve this problem, the invention has come up with the idea of computing a set of lengths on the image of the vessel to be fitted out, preferably a minimum length and a maximum length, so as to have knowledge of all the constraints that the stent will have to support. The invention shall be described in the context of the thoracic artery but it can also be applied to any other curved tubular structure.
According to the invention, on the tubular image of the vessel, the method starts by the plotting of lines applied to the tubular surface. Thus, a certain number of lines are plotted, evenly separated from one another as much as possible, that never intersect one another and support minimum torsion.
In practice, after having obtained a 3D digital acquisition of the image of the vessel, a central line of this vessel is sought. With the knowledge of this central line, two processing operations are performed. Firstly, a segmentation of the digital image is performed starting from this central line to find the position of the walls of the tubular vessel. Secondly, this central line is subdivided into a set of segments whose number will be all the greater as the dimensions of the vessel have to be approached with greater precision. In practice, it can be noted that, over a real length of 3 cm for a vessel, about 30 segments can be envisaged but the number of these segments may be greater or smaller.
Wall line portions corresponding to these segments are plotted on the walls of the vessel. The wall line portions are joined to one another and form total wall lines whose length is definitively measured. This mode of action gives knowledge of the maximum length, the minimum length, the mean length and many other items of information on the length of the vessel and therefore on the length of the stent to be positioned in the vessel. The choice of this stent may be more appropriate.